I think I found the problem. The text in, for example, David's post that you mention contained non-ASCII characters for certain characters (double quotation marks, single quotation marks, dashes, etc). This kind of thing happens, say, if someone copied text from Microsoft Word which uses "smart" quotes.

I went and modified the system so that it replaces instances of such non-ASCII characters as above with their regular ASCII counterparts (eg ", ', --). If you run into any other problem such as those that you saw previously, please let me know with the exact post and I'll take a look into it.